Akitaka Kimura

Room-Temperature Continuous-Wave Operation of InGaN Multi-Quantum-Well Laser Diodes Grown on an n-GaN Substrate with a Backside n-Contact

Continuous-wave operation at room-temperature has been demonstrated for InGaN multi-quantum-well (MQW) laser diodes (LDs) grown on low-dislocation-density n-GaN substrates with a backside n-contact. The current, current density and voltage at the lasing threshold were 144 mA, 10.9 kA/cm2 and 10.5 V, respectively, for a 3 µm wide ridge-geometry diode with high-reflection dielectric coated mirrors. Single-transverse-mode emission was observed in the far-field pattern of the LDs and the beam full width at half power in the parallel and perpendicular directions was 6° and 25°, respectively.

High-quality InGaN MQW on low-dislocation-density GaN substrate grown by hydride vapor-phase epitaxy

Abstract A low-dislocation-density thick GaN layer was successfully grown using selective-area HVPE growth combined with epitaxial lateral overgrowth. The InGaN MQWs fabricated on this thick GaN layer showed superior optical properties compared with that on a sapphire substrate. Mg diffusion, induced by threading dislocations, was greatly suppressed for the LEDs on the HVPE-grown GaN layer. The results clearly indicate that the HVPE-grown GaN substrate will be useful for achieving high-performance light-emitting devices.

Self-organized propagation of dislocations in GaN films during epitaxial lateral overgrowth

Dislocation propagation and defect evolution in GaN films formed by epitaxial lateral overgrowth (ELO) are examined by transmission electron microscopy. A novel effect that induces self-organized propagation of preexisting dislocations in ELO films is evaluated. This propagation forms dislocations into bundle structures along the stripes of masks used for ELO. The dislocation bundling gives rise to crystallographic tilting in the overgrown region on the mask and leads to a total reduction of threading dislocation density in the film.

Novel Ridge-Type InGaN Multiple-Quantum-Well Laser Diodes Fabricated by Selective Area Re-Growth on n-GaN Substrates

A novel ridge structure fabricated by selective-area epitaxial re-growth is proposed for InGaN multiple-quantum-well (MQW) laser diodes (LDs). This technique is capable of precisely controlling the active ridge width and height, thus enabling stable single tran sverse-mode operation. Together with a backside n-contact on a low-dislocation-density GaN substrate, this structure provides high productivity and performance for GaN-based blue-violet LDs. A stable fundamental transverse mode up to 40 mW was demonstrated for the certain range of ridge dimensions. The minimum aspect ratio of the far-field patterns (FFPs) was about 2.1 in the fabricated ridge-type InGaN MQW LDs.

Precise control of pn-junction profiles for GaN-based LD structures using GaN substrates with low dislocation densities

Abstract Doping profiles of pn-junctions for GaN-based laser diode structures were studied using SIMS and EBIC measurements. Mg impurity, a p-type dopant, was found to diffuse heavily into the n-type region of the order of 10 17 xa0cm −3 when a sample is grown on a sapphire substrate. This unwanted diffusion appears closely related to the presence of threading dislocations, and can be eliminated by using high-quality GaN substrates for epitaxial growth.

Temperature characteristics of a vertical-cavity surface-emitting laser with a broad-gain bandwidth

Temperature-insensitive characteristics are of great importance in implementing the actual applications of vertical-cavity surface-emitting lasers (VCSELs) because of the temperature change in the surroundings. To extend the operational temperature range of such lasers, we fabricated a VCSEL with a broad gain bandwidth. The active layers in VCSELs consist of multiple quantum wells (MQWs) with different bandgap energies. From the change in the threshold current, with temperature as a parameter, we found that the operational temperature range of a VCSEL with a broad gain bandwidth is more than 20/spl deg/C wider than that of conventional VCSELs, whose active layers consist of a single type of MQW. We demonstrate that the extended-gain bandwidth gives better temperature characteristics. In addition, we simulated the structure of the active layers, and the optimized structure resulted in a 1-mW light output power at less than 5 mA in a single transverse mode oscillation from 20-70/spl deg/C. >

X-Ray Rocking Curve Determination of Twist and Tilt Angles in GaN Films Grown by an Epitaxial-Lateral-Overgrowth Technique

X-ray rocking curve measurements have been carried out in order to investigate the twisting and tilting in hydride vapor phase epitaxy (HVPE) and metalorganic vapor phase epitaxy (MOVPE) GaN films fabricated by an epitaxial lateral overgrowth (ELO) technique. The twist angle is directly determined by means of grazing incidence angle X-ray diffraction. It is found that these angles in both MOVPE and HVPE films are significantly reduced by the ELO technique especially for the twist angles. In the MOVPE-ELO GaN film, the tilt and twist angles depend on the stripe direction of the mask pattern, which is closely related to the difference of the growth process.

Reflectance spectroscopy on GaN films under uniaxial stress

The uniaxial stress effects on valence band structures in GaN are investigated by reflectance spectroscopy. It is observed that the energy separation between A and B valence bands increases with the application of uniaxial stress in the c plane. The experimental results are analyzed on the basis of the k⋅p theory, and deformation potential D5 is determined as −3.3u2009eV. It is indicated that the uniaxial strain effect could be utilized for improving GaN-based laser performance.

Supersaturation-dependent step-behavior of InGaN grown by metal organic vapor phase epitaxy

Abstract The step-behavior of InGaN thin films grown on sapphire substrates by metal organic vapor phase epitaxy was studied using atomic force microscopy characterization. Spiral size and interstep distance decreased when the supersaturation of the group-III source was increased. Using this dependency and the Burton, Cabrera, and Frank model, the step energy was calculated to be 2.6xa0J/m 2 for InN in InGaN and 1.5xa0J/m 2 for GaN. InGaN spiral growth on sapphire substrates is caused by the specific large step energy of this material. Therefore, reducing dislocation densities in the epitaxial layers is most effective for reducing spiral densities. This was confirmed by growing InGaN on FIELO-GaN substrates, which have a low dislocation density.

Growth of InN by Chloride-Transport Vapor Phase Epitaxy

Hexagonal single crystalline InN film are grown on a hexagonal-GaN epitaxial layer in a GaAs(100) substrates by chloride-transport vapor phase epitaxy. It is found that lower temperatures for the upstream region of the reator, where indium chloride is generated, are crucial for InN growth. In addition, the use of inert gas such as nitrogen, is necessary for growth. Based on these results, the necessity of InCl3 as a source material is emphasized for InN growth. The grown film reveals a hexagonal structure whose c-axis orients to the GaAs B direction.